1. Field of the Invention
The present invention relates to a semiconductor integrated circuit which performs various kinds of operation processes, and particularly to what controls electricity consumption in it.
2. Description of Related Art
In the field of semiconductor integrated circuits, the operational speed of processors is becoming higher. Accordingly the frequency of a system clock signal is becoming higher and electricity consumption in the circuit is becoming larger. But, this high speed operation is not always needed in actual application processes. So, the system clock signal frequency is controlled.
A conventional semiconductor integrated circuit with this kind of control will be described according to FIG. 2.
The circuit in FIG. 2 comprises a microprocessor (MPU) 1, plural function modules 2i(i=1,2, . . . ,n), and a clock gear mechanism 3. The microprocessor is what controls the system overall, and each function module 2i performs a process such as communication control or graphic compression etc. The clock gear mechanism 3 is what divides the frequency of an inputted clock signal MCK according to the control by microprocessor 1.
The microprocessor 1, function modules 2i, and clock gear mechanism 3 are connected with a system bus 4. On the other hand, a system clock signal SCK generated by clock gear mec. 3 is provided to microprocessor 1 via clock signal line 5. Moreover, each function module 2i is provided with the system clock signal SCK via a disable mechanism 6i which comprises a register (REG) and an AND gate respectively.
The electric power Pw consumed in a semiconductor integrated circuit is represented in general by next formula.Pw=Σαi·βi·f
In this, αi represents current consumed when it flows through elements in function module 2i, βi represents toggling rate of elements in function module 2i per unit time, and f represents operating frequency.
Moreover, the maximum consumed power Pwmax needs to be designed according to next formula where Rj is thermal resistance of the package and Tjmax is junction limit temperature of transistor.Pwmax≦Tjmax/Rj
Therefore, in the state of operating all the function module 2i in FIG. 2, for the total power consumed not to exceed Pwmax, the frequency division value is set, so as to limit the frequency f of system clock signal SCK. In this limiting of frequency f, the toggling rate is assumed as “1”.
Moreover, when all modules 2i do not need simultaneous operation, the individual operation of each function module 2i is controlled by each disable mec.6j. And, system clock signal SCK is stopped providing for unnecessary function modules. Thus, electricity consumption is restrained instead frequency of system clock signal SCK is increased. That is, according to next formula, different operating frequency fi is provided to each function module 2iPw=Σαi·βi·fi
However, the conventional semiconductor circuit had next subject.
The maximum operating frequency fmax is set by microprocessor 1 according to the formula mentioned above, as toggling rate βi is let be “1”. But, a certain period of time is needed for βi to become “1”. When the value of βi comes near to “1”, almost all the elements repeat charging and discharging. To be such an operating state, a certain period of time is needed. Therefore, for this certain period of time, temp.Tj of transistor does not become Tj>Tjmax even if clock frequency is let be beyond fmax. Consequently, within this certain period of time, process ability can be temporarily raised up by setting clock frequency beyond maximum operating frequency temporarily. In spite of this, the circuit in FIG. 2 could never do this. For it could not exactly watch the actual temp.Tj.
Moreover, even if operation of function module 2i is restrained by the operating system of microprocessor 1, the user application program does not always accord with this restraint. Therefore, there is a possibility of operating too many function modules. And, it is threatened that the circuit may go in malfunction owing to overheat.